In the case of a mobile telephone system which functions in accordance with the time division multiple access (TDMA) principle, the transmission and reception time-slots are included in so-called frames. One frame can contain, for instance, 10 time-slots, of which some are used to transmit bursts of speech/data signals, where each burst takes-up substantially a complete time-slot. Time-slots can be used to transmit synchronizing information, etc., for example. The mobile station can also utilize vacant or unoccupied time-slots for measuring purposes, e.g. for measuring signal strength over a radio channel other than the channel assigned to the mobile station. The signal strength of channels which belong to another base station can be measured while a time-slot is vacant when so-called hand-over shall take place.
Transmission and reception in the various time-slots can take place either simultaneously or in time-slots which are separated in time. In the former case, reception takes place in a reception time-slot at the same time as transmission takes place in a transmission time-slot, i.e. the two time-slots coincide complete or partially. This requires the provision of a so-called duplex filter in the mobile station, in order to separate mututally transmitted and received signals which have mutually different carrier frequencies.
In the latter of the aforesaid two cases, in which the transmission and reception time-slots are separated in time, the reception circuits of the mobile station are fully disengaged during transmission, and vice versa. No duplex filter is required in this latter case. An example of one such mobile telephone system is described in U.S. Pat. No. 4,817,089 which operates in semiduplex. This known system includes separate frequency synthesizers which are operative to set the carrier frequencies in the transmitting and receiving directions. In known systems which use duplex filters to enable signals to be received and transmitted simultaneously, there is obtained a certain rest or idle period which can be used to determine signal strength.